Add reusable docker workflow.

.gitea/workflows/docker.yaml

@@ -0,0 +1,34 @@
+# Reusable workflow, callable from other repos, to build and possibly push a
+# container image. The container must contain a file /tag which contains the
+# desired image tag.
+
+on:
+  workflow_call:
+    inputs:
+      image_name:
+        description: Image name, like docker.io/me/my_image
+        required: true
+        type: string
+      push:
+        description: True if the image should be pushed after building
+        required: false
+        default: false
+        type: boolean
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: build
+      run: docker build -t ${{ inputs.image_name }}:${{ github.run_number }}
+    - name: push
+      if: ${{ inputs.push }}
+      run: |
+        docker create --name ${{ inputs.image_name }}_${{ github.run_number }} ${{ inputs.image_name }}:${{ github.run_number }}
+        docker export -o /tmp/image ${{ inputs.image_name }}_${{ github.run_number }}
+        docker rm ${{ inputs.image_name }}_${{ github.run_number }}
+        docker rmi ${{ inputs.image_name }}:${{ github.run_number }}
+        tar -x -C /tmp -f /tmp/image tag
+        tag=$(cat /tmp/tag)
+        docker tag ${{ inputs.image_name }}:${{ github.run_number }} ${{ inputs.image_name }}:$tag
+        docker push ${{ inputs.image_name }}:$tag